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538f19526e
With dynamic function tracer, by default, _mcount is defined as an
"empty" function, it returns directly without any more action . When
enabling it in user-space, it will jump to a real tracing
function(ftrace_caller), and do the real job for us.
Differ from the static function tracer, dynamic function tracer provides
two functions ftrace_make_call()/ftrace_make_nop() to enable/disable the
tracing of some indicated kernel functions(set_ftrace_filter).
In the -v4 version, the implementation of this support is basically the same as
X86 version does: _mcount is implemented as an empty function and ftrace_caller
is implemented as a real tracing function respectively.
But in this version, to support module tracing with the help of
-mlong-calls in arch/mips/Makefile:
MODFLAGS += -mlong-calls.
The stuff becomes a little more complex. We need to cope with two
different type of calling to _mcount.
For the kernel part, the calling to _mcount(result of "objdump -hdr
vmlinux"). is like this:
108: 03e0082d move at,ra
10c: 0c000000 jal 0 <fpcsr_pending>
10c: R_MIPS_26 _mcount
10c: R_MIPS_NONE *ABS*
10c: R_MIPS_NONE *ABS*
110: 00020021 nop
For the module with -mlong-calls, it looks like this:
c: 3c030000 lui v1,0x0
c: R_MIPS_HI16 _mcount
c: R_MIPS_NONE *ABS*
c: R_MIPS_NONE *ABS*
10: 64630000 daddiu v1,v1,0
10: R_MIPS_LO16 _mcount
10: R_MIPS_NONE *ABS*
10: R_MIPS_NONE *ABS*
14: 03e0082d move at,ra
18: 0060f809 jalr v1
In the kernel version, there is only one "_mcount" string for every
kernel function, so, we just need to match this one in mcount_regex of
scripts/recordmcount.pl, but in the module version, we need to choose
one of the two to match. Herein, I choose the first one with
"R_MIPS_HI16 _mcount".
and In the kernel verion, without module tracing support, we just need
to replace "jal _mcount" by "jal ftrace_caller" to do real tracing, and
filter the tracing of some kernel functions via replacing it by a nop
instruction.
but as we have described before, the instruction "jal ftrace_caller" only left
32bit length for the address of ftrace_caller, it will fail when calling from
the module space. so, herein, we must replace something else.
the basic idea is loading the address of ftrace_caller to v1 via changing these
two instructions:
lui v1,0x0
addiu v1,v1,0
If we want to enable the tracing, we need to replace the above instructions to:
lui v1, HI_16BIT_ftrace_caller
addiu v1, v1, LOW_16BIT_ftrace_caller
If we want to stop the tracing of the indicated kernel functions, we
just need to replace the "jalr v1" to a nop instruction. but we need to
replace two instructions and encode the above two instructions
oursevles.
Is there a simpler solution? Yes! Here it is, in this version, we put _mcount
and ftrace_caller together, which means the address of _mcount and
ftrace_caller is the same:
_mcount:
ftrace_caller:
j ftrace_stub
nop
...(do real tracing here)...
ftrace_stub:
jr ra
move ra, at
By default, the kernel functions call _mcount, and then jump to ftrace_stub and
return. and when we want to do real tracing, we just need to remove that "j
ftrace_stub", and it will run through the two "nop" instructions and then do
the real tracing job.
what about filtering job? we just need to do this:
lui v1, hi_16bit_of_mcount <--> b 1f (0x10000004)
addiu v1, v1, low_16bit_of_mcount
move at, ra
jalr v1
nop
1f: (rec->ip + 12)
In linux-mips64, there will be some local symbols, whose name are
prefixed by $L, which need to be filtered. thanks goes to Steven for
writing the mips64-specific function_regex.
In a conclusion, with RISC, things becomes easier with such a "stupid"
trick, RISC is something like K.I.S.S, and also, there are lots of
"simple" tricks in the whole ftrace support, thanks goes to Steven and
the other folks for providing such a wonderful tracing framework!
Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: Nicholas Mc Guire <der.herr@hofr.at>
Cc: zhangfx@lemote.com
Cc: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/675/
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
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| .. | ||
| cpufreq | ||
| .gitignore | ||
| 8250-platform.c | ||
| asm-offsets.c | ||
| binfmt_elfn32.c | ||
| binfmt_elfo32.c | ||
| branch.c | ||
| cevt-bcm1480.c | ||
| cevt-ds1287.c | ||
| cevt-gt641xx.c | ||
| cevt-r4k.c | ||
| cevt-sb1250.c | ||
| cevt-smtc.c | ||
| cevt-txx9.c | ||
| cpu-bugs64.c | ||
| cpu-probe.c | ||
| csrc-bcm1480.c | ||
| csrc-ioasic.c | ||
| csrc-powertv.c | ||
| csrc-r4k.c | ||
| csrc-sb1250.c | ||
| early_printk.c | ||
| entry.S | ||
| ftrace.c | ||
| genex.S | ||
| gpio_txx9.c | ||
| head.S | ||
| i8253.c | ||
| i8259.c | ||
| init_task.c | ||
| irq-gic.c | ||
| irq-gt641xx.c | ||
| irq-msc01.c | ||
| irq-rm7000.c | ||
| irq-rm9000.c | ||
| irq.c | ||
| irq_cpu.c | ||
| irq_txx9.c | ||
| kgdb.c | ||
| kspd.c | ||
| linux32.c | ||
| machine_kexec.c | ||
| Makefile | ||
| mcount.S | ||
| mips-mt-fpaff.c | ||
| mips-mt.c | ||
| mips_ksyms.c | ||
| module.c | ||
| octeon_switch.S | ||
| proc.c | ||
| process.c | ||
| ptrace.c | ||
| ptrace32.c | ||
| r4k_fpu.S | ||
| r4k_switch.S | ||
| r2300_fpu.S | ||
| r2300_switch.S | ||
| r6000_fpu.S | ||
| relocate_kernel.S | ||
| reset.c | ||
| rtlx.c | ||
| scall32-o32.S | ||
| scall64-64.S | ||
| scall64-n32.S | ||
| scall64-o32.S | ||
| setup.c | ||
| signal-common.h | ||
| signal.c | ||
| signal32.c | ||
| signal_n32.c | ||
| smp-cmp.c | ||
| smp-mt.c | ||
| smp-up.c | ||
| smp.c | ||
| smtc-asm.S | ||
| smtc-proc.c | ||
| smtc.c | ||
| spram.c | ||
| stacktrace.c | ||
| sync-r4k.c | ||
| syscall.c | ||
| time.c | ||
| topology.c | ||
| traps.c | ||
| unaligned.c | ||
| vmlinux.lds.S | ||
| vpe.c | ||
| watch.c | ||